The virus has usually left the body within two weeks but in rare cases it lies dormant in the brain for years.

If the virus reactivates the result can be SSPE which leads to memory loss, mood changes, muscle spasms, occasional blindness and sometimes the patient will become comatose.

Patients with SSPE die as a result of fever, heart failure, or the brain’s inability to control vital organs.

Scientists at the University of California, who carried out the study on children who had measles during an outbreak in California around 1990 over an 18-year period, are reportedly alarmed at the discovery.

They stressed that the only certain way to avoid SSPE is universal ­vaccination. If enough of the population was immunised the spread of measles would be contained enough to protect those who are too young to have the vaccine.

To protect children who are too young to have the vaccine parents should avoid travelling to endemic areas.